The present invention relates to radio receivers, transceivers, systems and methods employing wireless digital communications using ultra wide bandwidth (UWB) signaling techniques, and other communication waveforms. More particularly, the present invention relates to multi-mode communication systems in which one of the modes employs UWB waveforms.
UWB waveforms are used in a form of communication in which energy is spread in frequency over a much greater bandwidth than with conventional narrowband communication systems such as television broadcast systems, or even traditional spread-spectrum communication systems. For a general discussion of UWB communications as well as other UWB systems, see the above-identified related patent application documents.
FIG. 1 is a generalized spectral plot of energy verses frequency showing how UWB compares with conventional communication schemes. In particular, FIG. 1 shows a conventional narrowband communications signal (NB3) 105, a conventional spread spectrum communications signal (SS2) 110, and a UWB communications signal (UB1) 115.
As shown in FIG. 1, a conventional narrowband communications signal 105 occupies a relatively narrow frequency. In this category, a television signal occupies a relatively large bandwidth and is representative of one of the widest bandwidth signals that is still characterized as a conventional narrowband signal. The signal spectrum from a conventional spread-spectrum signal 110 occupies a greater bandwidth than a conventional narrowband communications signal 105, but at a lower power spectral density (PSD), i.e., at a lower amount of energy per hertz.
A UWB signal 115 occupies a much greater bandwidth than either a conventional narrowband communications signal (NB3) 105 or a conventional spread spectrum communications signal (SS2) 110. However, as seen in FIG. 1, a UWB signal 115 also has a much lower PSD than either of these other signal types. This can lead to several problems.
First, as recognized by the present inventor, because the bandwidth is so broad in a UWB receiver, it is common to employ a direct conversion receiver architecture. However, when there is local oscillator leakage (radiated or conducted), that leakage often manifests itself either as direct radiated emissions or a leak that is in some way coupled into the front-end circuitry so as to contaminate the intended energy coupled into the signal path. Moreover, in direct conversion receivers (i.e., those without intermediate frequencies, or with a single mixer for converting from RF to baseband), there is the additional problem of the local oscillator producing a radiated emission that may be coupled back through the receiving antenna and serve as a self-jamming signal. This local oscillator may radiate emissions that fall “in band” with the received signal and are coupled through the receiving antenna, being detected and much stronger than the desired signal.
One conventional technique for eliminating this problem is to employ a significant amount of shielding around the local oscillator circuitry to avoid direct radiation from the circuitry reaching the receiving antenna. Another is to minimize the opportunities for feedback loops within the receiver, thus limiting possible occurrences of self-interference.
A common way of mitigating the resulting interference is to assume that it is unchanging, and subtracting the error that is likewise unchanging. In addition, however, a problem that arises when the emissions are radiated is that the emissions can first reflect off moving objects near the antenna, such as individuals or devices, and then be coupled back into the receiver's antenna. When these reflections are present, the energy of the reflected emissions tends to vary in magnitude and phase. Accordingly, the unwanted self-noise is often not constant, but variable in time, causing a bias level of the receiver's detector to vary. This makes self-noise detection and mitigation more complex and increases the difficulty in obtaining satisfactory performance from the device.
Moreover, the output of a conversion mixer, which is used to perform direct signal conversion, will contain both the intended signal as well as the reflected signal. These two signals may be added coherently, giving rise to a bias term. Because the unintended signals may come from reflections off moving objects, the bias term is not steady, but rather “noisy.”
Second, it is desirable to build a multi-mode radio that can easily operate in multiple modes, including both ultra wide bandwidth signaling and various narrowband-signaling signaling schemes. For example, it would be advantageous to get economies of scale, to mass produce a radio that could operate not only in a UWB mode, but also in, for example, an IEEE 802.11b mode, or an IEEE 802.11.a mode, or an IEEE 802.15.1 mode, or an IEEE 802.15.3 mode, etc. so that users could operate in a multitude of various environments without requiring a different piece of equipment for each one. It would also be advantageous for the radio to be software programmable, so that as new communications equipment is introduced, the radio could be programmed to receive the signal without requiring an entirely new radio transmitter, receiver or transceiver to be developed.